1. Field of the Invention
The present invention relates to a method and apparatus for feedback control of the air-fuel ratio in an internal combustion engine.
2. Description of the Prior Art
As measures taken against exhaust gas pollution and fuel consumption, a lean burn system has recently been developed. According to this lean burn system, a lean mixture sensor is provided for generating an analog current in proportion to the air-fuel mixture on the lean side in an exhaust pipe of an engine. Thus, the feedback of the air-fuel ratio of the engine can be controlled by using the analog output of the lean mixture sensor, thereby attaining a voluntary air-fuel ratio on the lean side.
In the above-mentioned lean burn system, however, although it is advantageous in view of fuel consumption to control the feedback of the air-fuel ratio, so that it may shift on the leaner side, the feedback controlled value of the air-fuel ratio is affected by the characteristics of the lean mixture sensor, the exhaust gas composition characteristics and the like. That is, the feedback controlled value often deviates from a desired value as a result of the individual differences in the control characteristics of the parts of the engine due to aging of the engine or due to environmental changes. Therefore, if the controlled value of the air-fuel ratio is very close to a misfire limit, the air-fuel ratio may deviate into the misfire limit, thereby inviting misfiring of the engine. In order to avoid such misfiring, the controlled value of the air-fuel ratio is actually within a stable region sufficiently apart from the misfiring limit. Nevertheless, the controlled air-fuel ratio may be near the misfire limit due to aging of the engine or due to environmental changes, thus inviting misfiring of the engine and reducing the drivability.